1. Field of the Invention
This invention relates to an improved process for production of polyarylene sulfides and, in particular, to such a process which enables efficient production of a highly pure polyarylene sulfide having a decreased content of by-product oligomers and a relatively increased molecular weight.
Polyarylene sulfides including polyphenylene sulfide, abbreviated as PPS hereinafter, as a typical example have excellent heat and chemical resistance properties and, hence, have attracted a good deal of attention as materials useful for producing parts in electrical and electronic instruments, automotive parts and the like. The materials may be formed or molded, for example, by injection, extrusion or the like technique into various molded or shaped parts, films, sheets, fibers and the like. These molded or shaped products have been used in a wide range of applications where high heat and chemical resistance properties are required.
2. Prior Art
Various processes for production of the polymeric materials are known. A typical method is disclosed, for example, in Japanese Patent Publication (KOKOKU) No. 45-3368, which method comprises reacting a dihaloaromatic compound with a sulfur source, such as sodium sulfide, in an organic solvent, such as N-methyl pyrrolidone.
However, the polymers produced such a method exhibit a low molecular weight and are contaminated with various by-products such as linear and cyclic oligomers and decomposition products of the solvent. Such a contaminated low molecular polymer product tends to emit heavily gases when it undergoes hot shaping and forming, and thus to make the shaping and forming procedure difficult and to result in final products having poor strength properties.
Japanese Patent Public Disclosure (KOKAI) No. 59-98133 discloses a modified process of PPS synthesis in which the reaction system comprising a dihaloaromatic compound, a sulfur source and an organic amide polar solvent is pretreated to adjust its water-content to a range of 0.3 to 0.95 moles per mole of the sulfur source, such as sodium sulfide, before the synthesis polymerization reaction. However, since the sulfur source as supplied is generally in a hydrous form and must be thermally dehydrated for a prolonged period of time in the organic amide polar solvent prior to the polymerization, both the sulfur source and the solvent are subject to decomposition or degradation during the dehydration stage, which will lead to generation of by-products, such as oligomers and, thus, to production of PPS of a poor purity in the subsequent PPS synthesis stage.
In order to reduce the content of oligomer impurities in polyarylene sulfides, it has been proposed to separate the reaction mixture, after synthesis of polyarylene sulfide, into the solids and liquid fractions while the mixture remaining hot and further washing the recovered polymeric product with a heated solvent which is the same as the used reaction solvent; see, for example, Japanese Patent Public Disclosure (KOKAI) No. 62-232437. However, this approach can achieve the intended reduction in the oligomer content of the product polymer with the added, complicated procedure for removal of oligomers and will result in an unacceptably decreased productivity of the process.